/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace Lucene.Net.Codecs.Bloom
{

    using System;
    using Util;

    /// <summary>
    /// This is a very fast, non-cryptographic hash suitable for general hash-based
    /// lookup. See http://murmurhash.googlepages.com/ for more details.
    ///
    /// The C version of MurmurHash 2.0 found at that site was ported to Java by
    /// Andrzej Bialecki (ab at getopt org).
    ///
    ///  The code from getopt.org was adapted by Mark Harwood in the form here as one of a pluggable choice of 
    /// hashing functions as the core function had to be adapted to work with BytesRefs with offsets and lengths
    /// rather than raw byte arrays.  
    /// 
    /// @lucene.experimental
    /// </summary>
    public sealed class MurmurHash2 : HashFunction
    {

        public static readonly MurmurHash2 INSTANCE = new MurmurHash2();

        private MurmurHash2()
        {
        }

        public static int Hash(byte[] data, uint seed, int offset, int len)
        {
            int m = 0x5bd1e995;
            int r = 24;
            int h = (int)(seed ^ (long)len);
            int len_4 = len >> 2;
            for (int i = 0; i < len_4; i++)
            {
                int i_4 = offset + (i << 2);
                int k = data[i_4 + 3];
                k = k << 8;
                k = k | (data[i_4 + 2] & 0xff);
                k = k << 8;
                k = k | (data[i_4 + 1] & 0xff);
                k = k << 8;
                k = k | (data[i_4 + 0] & 0xff);
                k *= m;
                k ^= k >> r;
                k *= m;
                h *= m;
                h ^= k;
            }
            var len_m = len_4 << 2;
            var left = len - len_m;
            if (left != 0)
            {
                if (left >= 3)
                {
                    h ^= data[offset + len - 3] << 16;
                }
                if (left >= 2)
                {
                    h ^= data[offset + len - 2] << 8;
                }
                if (left >= 1)
                {
                    h ^= data[offset + len - 1];
                }
                h *= m;
            }
            h ^= h >> 13;
            h *= m;
            h ^= h >> 15;
            return h;
        }

        /// <summary>
        /// Generates 32 bit hash from byte array with default seed value.
        /// </summary>
        /// <param name="data">byte array to hash</param>
        /// <param name="offset">the start position in the array to hash</param>
        /// <param name="len">length of the array elements to hash</param>
        /// <returns>32 bit hash of the given array</returns>
        public static int Hash32(byte[] data, int offset, int len)
        {
            return Hash(data, 0x9747b28c, offset, len);
        }

        public override int Hash(BytesRef br)
        {
            return Hash32((byte[])(Array)br.Bytes, br.Offset, br.Length);
        }

    }
}
